Molecular characterization of methanol-soluble BrC using 1H NMR fingerprinting and high-resolution mass spectrometry.

Brown carbon (BrC) is the light-absorbing fraction of organic aerosols that can affect atmospheric photochemistry and influence regional and global climate. BrC can be emitted directly from incomplete combustion or formed through multiphase reactions. BrC consists of both water-soluble and methanol-soluble fractions (Me-BrC), with the Me-BrC fraction exhibiting greater absorption compared to the aqueous fraction owing to greater extraction potential. It is important to characterize the molecular composition of BrC to gain a better understanding of the link between BrC chromophores and their light absorption properties. In this study, proton nuclear magnetic resonance (¹H NMR) and high-performance liquid chromatography (HPLC) coupled with a diode array detector (DAD) and a time-of-flight mass spectrometer (TOF-MS) with an electrospray ionisation (ESI) source are used to comment on the molecular composition of Me-BrC. For this purpose, daytime and nighttime PM_2.5 samples collected from April 2019 to February 2020 in a rural area in the eastern Indo-Gangetic Plain (IGP) were used.  

Both ESI- and ESI+ modes were utilized due to their respective affinity towards acidic and basic functional groups and about 500-2000 molecules were classified into four major compound classes: CHO, CHON, CHONS and CHOS. Among these, CHO- (25-32%) was the most abundant group followed by CHONS- (25-30%), CHON- (21-28%) and CHOS- (13-17%) in the negative mode whereas CHON+ (42-50%) showed the highest abundance in the positive mode followed by CHONS+ (30-38%) and CHO+ (6-14%) and CHOS+ (5-13%). ¹H-NMR showed dominance of saturated oxygenated compounds (H-C-O) for all seasons, which is consistent with the high O/C ratio and OSc (oxidation state), with increasing Ar-H and hence double bond equipment (DBE) towards winter. Higher O/C (>0.3) and lower DBE of CHON+ and CHONS+ showed their significant contribution towards H-C-O type of BrC along with CHO-. CHON species can be formed by atmospheric reactions involving NOx (oxidised N-containing groups (O/C: 0.4, H/C: 1.5)) or NH₃ (reduced N-containing groups (O/C: 0.2, H/C: 1.9)) with CHON- containing more organic nitrate moieties (−(O)NO₂); (O/N >3)). Lower CHON towards the night time cross all seasons corresponds to the lower relative percentage of the H–C–C= type in NMR. High CHONS+ suggest presence of nitroxy organo-sulfate (formed via photooxidation of BVOCs under high NOx) or come from coal combustion. CHOS+ showed greater fraction of high‐molecular weight organosulfate compounds (low DBE or high H/C) which increased towards the winter. Also CHOS+ with high DBE (>4) with O/C (<0.4) suggests presence of reduced S‐containing compounds which are more likely to be originated from primary emissions. These findings provide the first insights into the chromophoric composition of the Indo-Gangetic Plain outflow.